1. Field of the Invention
This invention relates to a power screed for smoothing concrete and a method of operating said screed where the screed has adjustable vertical support means to account for uneveness in a supporting surface on which the concrete is being laid in order t control the level of the screed as desired.
2. Description of the Prior Art
It is known to have power screeds that are used during the laying of concrete, particularly in the laying of concrete floors, roadways, ramps, walkways and the like. The screeds have an elongated shape two sides, a front and a rear and are designed to be placed transversely relative to a width of concrete that is being laid. The screeds are self-propelled and vibrate in a forward direction along the width of concrete to smooth and level the concrete. Concrete is poured in front of the screed and roughly levelled using rakes or other suitable means before being contacted by the screed. As the screed passes over the roughly levelled concrete, it adjusts the level of the concrete to a predetermined height and smooths it so that concrete at a rear of the screed is smooth. With many of the known screeds, the level of the concrete is determined by bulkheads or forms that are constructed along either side of an area where a width of concrete is to be laid. One side of the screed rests on one bulkhead and the other side of the screed rests on the other bulkhead. As the concrete is poured, the screed moves forward to smooth out the concrete so that an upper surface of the concrete is at the same level as an upper surface of the bulkheads and a lower surface of the screed. When laying a concrete floor, for example, the bulkheads must be installed in several parallel rows, each row having a width that is smaller than the width of the screed. The screed is then placed onto the bulkheads with one side of the screed resting on one bulkhead and the other side of the screed resting on the remaining bulkhead. When concrete floors are poured in this manner, the concrete is poured in alternate rows. The concrete in those rows is allowed to set and then the bulkheads are removed and the concrete is poured in the remaining rows using the rows that have already set to support the screed in place of the bulkheads. The installation of the bulkheads is a time consuming and expensive process as the surface on which the concrete is to be laid is uneven. Also, the bulkhead system does not provide a sufficient degree of versatility. An upper surface of the bulkheads must be made to correspond to an upper surface of the concrete to be poured. When it is desired to vary a level of the concrete, for example, in the area of a drain, difficulties can be encountered. If the level of the screed is to be lowered the bulkheads must be sloped to the desired lower level. Another known power screed is suspended by a boom from a self-propelled motor vehicle. The level of this screed is controlled by lasers but the screed suffers from disadvantages in that: it is extremely expensive; it can be difficult to manoeuver in restricted areas because of its large size; it can only smooth concrete in an area of approximately twelve feet in width by twenty feet in length without stopping to relocate the motor vehicle; the screed itself must have a relatively narrow width (e.g. twelve feet) to keep the size of the entire machine within reason; or, it cannot be added to an existing screed.